Field of the Invention
Embodiments of the present invention generally relate to wireless networks and in particular, to seamless roaming between access points in different IP subnets in a wireless network.
The use of wireless devices (also referred to herein as “mobile units”) is becoming more prevalent due, in part, to the versatility and mobility of such devices. In many practical applications, wireless devices use wireless networks to communicate with each other. To communicate with hosts on a wired network (e.g., the Internet), the wireless devices need to be connected to a wireless local area network (WLAN) using, for example, an IEEE 802.11 standard protocol wireless network (hereinafter referred to as an “802.11 wireless network” or “802.11 WLAN”) and devices known as access points (APs).
An AP or wireless access point (WAP) is equipped with an interface that then connects the wireless devices to a wired network like the Ethernet. An AP may also include a wireless interface that connects wireless communication devices to create a wireless network. Mobile units can access the Internet, for example, via a device called an access router (AR), which forwards packets from mobile units to hosts on the wired network through the AP's.
The coverage area of an AP is limited (e.g., about one hundred to two hundred feet indoors). Therefore, multiple APs are needed to cover a large area. There are two approaches for deploying multiple APs. The first is where the APs function as link-layer (layer-2) bridges. The second is where the APs function as network-layer (layer-3) routers. In the first approach (layer-2 approach), the AR, the APs and all the mobile units are on the same IP subnet. In the second approach (layer-3), which is more flexible in terms of deployment, the APs can reside on different IP subnets, function as IP routers to the mobile units and communicate with the AR using IP packets instead of link-layer frames.
One of the main design issues in the deployment of 802.11 WLANs is the issue of seamless roaming support for mobile units within the wireless network. Mobility has to be supported without any, or at least a minimum of, service disruptions. For example, application level sessions should not be disrupted during handoffs between APs that provide link-layer connectivity to the mobile unit. If the APs are within the same IP subnet, a layer-2 handoff mechanism, inherent in 802.11 WLANs, would be sufficient to handle seamless mobility. If the APs are attached to different IP subnets, however, layer-3 handoff mechanisms would need to be employed to support the seamless mobility.
Layer-2 mechanisms have limitations. For instance, deployment in a wide area, linking every AP to one cable or one network presents an issue. It is more desirable to place the APs in the IP (or network) level in order to attach the APs to different IP sub-networks, allowing for more versatility. For one network to talk to another network, the handoff must be at the IP level (i.e., layer-3).
Known mechanisms for layer-3 mobility exist. For example, one layer-3 mobility mechanism is called Mobile-IP. However, Mobile-IP and related mechanisms are tailored towards nomadic connectivity and not for continuous mobility and frequent handoffs between APs that would occur in an 802.11 WLAN environment. Furthermore, Mobile-IP support is not widely available at the present time. For example, mobile units running MS Windows or Linux operating systems require the installation of additional software to use Mobile-IP.
Therefore, there is a need in the art for a seamless roaming support for 802.11 WLANs using a level-3 handoff scheme that can support frequent handoffs between APs and does not necessarily require installing additional software on the mobile units.